Complexing agent for displacement tin plating

Abstract

An environmentally innocuous effective replacement for thiourea is disclosed for use as a complexing agent in displacement plating processes in which the plating solution is applied to the substrate surface to be plated by immersion or by spraying, cascading, pouring and the like. The replacement complexing agent is an imidazole-2-thione compound having the formula: ##STR1## wherein A and B are the same or different --R--Y groups, wherein R is linear, branched or cyclic alkenyl group containing 1 to 12 carbon atoms and Y is a hydrogen, halogen, cyano, vinyl, phenyl, or ether moiety. Of this class of compounds, 1-methyl-3-propyl-imidazole-2-thione is preferred for immersion tin plating. This class of complexing agents is particularly useful in spray displacement tin plating for the manufacture of printed circuit boards wherein free tin metal is added to the plating solution.

What is claimed is: 
     
       1. An aqueous plating solution for the displacement plating of a substrate metal surface with an other metal comprising: (i) a metal ion of a free metal, wherein the free metal is different from the metal of the substrate surface;   (ii) a complexing agent which is a an imidazole-2-thione compound of the formula: ##STR10##  wherein A and B are the same or different --R--Y groups, wherein R is linear, branched or cyclic alkenyl group containing 1 to 12 carbon atoms and Y is a hydrogen, halogen, cyano, vinyl, phenyl, or ether moiety and   (iii) an acid.   
     
     
       2. The aqueous plating solution of claim 1 wherein the metal ion is a cation which is present in its lowest oxidation state and the imidazole-2-thione compound is an unsymmetrical 1,3-dialkylimidazole-2-thione wherein A is methyl or ethyl, and when A is methyl, B is a C 3  to C 6  alkyl or cycloalkyl group, and when A is ethyl, B is a C 4  to C 6  alkyl or cycloalkyl group. 
     
     
       3. The aqueous plating solution of claim 2 wherein the complexing agent is 1-methyl-3-propylimidazole-2-thione having the formula: ##STR11## 
     
     
       4. The aqueous plating solution of claim 2 wherein the aqueous plating solution contains one or more metal salts, the metal of the metal salt being selected from the group consisting of tin, germanium, lead, mercury, silver, indium, gold and palladium, wherein the metal ion of the metal salt is present in any oxidation state. 
     
     
       5. The aqueous plating solution of claim 4 wherein the cation is tin(II) and the metal of the metal salt is lead. 
     
     
       6. The aqueous plating solution of claim 1 wherein the the aqueous plating solution is maintained under an inert atmosphere which is substantially free of oxygen. 
     
     
       7. The aqueous plating solution of claim 1 wherein the aqueous plating solution contains therein a free metal (iv) which is the free metal of the metal ions (i). 
     
     
       8. The aqueous plating solution of claim 7 wherein the free metal (iv) is a metal of group IVA; VB; VIB; VIIB; VIII; IB; IIB and IIIA of the Periodic Table of the Elements. 
     
     
       9. The aqueous plating solution of claim 8 wherein the free metal (iv) is taken from the group consisting of tin, lead, mercury, nickel, gold, silver, indium, germanium, palladium, and mixtures thereof. 
     
     
       10. The aqueous plating solution of claim 1 wherein the substrate metal surface is copper or a copper alloy. 
     
     
       11. The aqueous plating solution of claim 1 wherein the metal ion (i) is a metal ion of a water soluble metal salt based on metals of group IVA; VB; VIB; VIIB; VIII; IB; IIB and IIIA of the Periodic Table of the Elements. 
     
     
       12. The aqueous plating solution of claim 11 wherein the metal of the water soluble metal salt is taken from the group consisting of tin, lead, mercury, nickel, gold, silver, indium, germanium, palladium and mixtures thereof. 
     
     
       13. The aqueous plating solution of claim 1 wherein the metal ion (i) is tin(II). 
     
     
       14. The aqueous plating solution of claim 1 wherein the acid (iii) is an inorganic acid based on sulfur, phosphorous, the halogens or mixtures thereof. 
     
     
       15. The aqueous plating solution of claim 14 wherein the acid (iii) is sulfuric acid or sulfamic acid. 
     
     
       16. The aqueous plating solution of claim 14 wherein the acid (iii) is a mixture of sulfuric acid and hypophosphorous acid. 
     
     
       17. The aqueous plating solution of claim 1 wherein the acid (iii) is an organic monocarboxylic or dicarboxylic acids having one to six carbon atoms. 
     
     
       18. The aqueous plating solution of claim 17 wherein the organic acid is taken from the group consisting of formic acid, acetic acid, malic acid, maleic acid, and mixtures thereof. 
     
     
       19. The aqueous plating solution of claim 1 wherein the aqueous plating solution contains a cosolvent. 
     
     
       20. The aqueous plating solution of claim 19 wherein the cosolvent is a solvent taken from the group consisting of alcohols, glycols, alkoxyalkanols, ketones and aprotic polar solvents. 
     
     
       21. The aqueous plating solution of claim 1 wherein the aqueous plating solution contains an additive selected from the group consisting of a urea compound, a reducing agent, a chelating agent, a surfactant, and mixtures thereof. 
     
     
       22. The aqueous plating solution of claim 1 wherein the substrate metal surface is the surface of electrically conductive copper circuitry adhered to at least one surface of a dielectric layer support with the circuitry having a thickness of at least 4 microns. 
     
     
       23. A process for displacement plating a substrate metal surface with an other metal comprising the steps: (a) providing a reservoir of aqueous plating solution comprising; (i) a cation of a free metal, which is present in its lowest oxidation state wherein the free metal is different from the metal of the substrate surface;   (ii) a complexing agent which is a compound of the formula: ##STR12##  wherein A is methyl or ethyl, and when A is methyl, B is a C 3  to C 6  alkyl or cycloalkyl group, and when A is ethyl, B is a C 4  to C 6  alkyl or cycloalkyl group;   (iii) an acid; and   (iv) a free metal which is the free metal of the cation present in its lowest oxidation state (i);     (b) directing a stream of the aqueous plating solution onto the substrate metal surface; whereby a portion of the cations of (i) are oxidized to ions in a higher oxidation state, and another portion of the cations of (i) are reduced to free metal, wherein said reduced free metal displaces surface substrate metal which is oxidized to an ion and complexed with the complexing agent to form a substrate metal ion complex dissolved in the reacted aqueous plating solution at the surface of the substrate metal; and   (c) returning the plating solution to the reservoir whereby at least a portion of the metal ions present in their higher oxidation state are reacted with free metal (iv) to form cations present in their lowest oxidation state to replenish the aqueous plating solution.   
     
     
       24. The process of claim 23 wherein the ratio of the surface area of the free metal (iv), to the volume of the aqueous plating solution is at least 4 in 2  /gallon (6.8 cm 2  /liter). 
     
     
       25. The process of claim 23 wherein in step (c), the portion of the formed cations present in their lowest oxidation state is twice the portion of the metal ions present in their higher oxidation state which reacted with the free metal (iv). 
     
     
       26. The process of claim 23 wherein the substrate metal surface is the surface of electrically conductive copper circuitry adhered to at least one surface of a dielectric layer support with the circuitry having a thickness of at least 4 microns and wherein the cation present in its lowest oxidation state (i) is tin(II) ion, the complexing agent (ii) is 1-methyl-3-propylimidazole-2-thione , the acid (iii) is sulfuric acid, the free metal (iv) is tin, the metal ion present in its higher oxidation state is tin(IV) ion, and the complexed substrate metal ion is a copper complex of 1-methyl-3-propylimidazole-2-thione. 
     
     
       27. The process of claim 23 wherein the stream of the aqueous plating solution is directed onto the substrate metal surface as a spray of the solution. 
     
     
       28. The process of claim 23 wherein the stream of the aqueous plating solution is directed onto the substrate metal surface as a cascade of the solution. 
     
     
       29. A process for immersion plating a substrate metal surface with an other metal comprising the steps: (a) providing a reservoir of aqueous plating solution comprising; (i) a metal ion of a free metal, wherein the free metal is different from the metal of the substrate surface;   (ii) a complexing agent which is a 1,3-dialkylimidazole-2-thione compound of the formula: ##STR13##  wherein A and B each individually is an alkyl or cycloalkyl group containing one to six carbon atoms; and   (iii) an acid;     (b) immersing the substrate metal surface into the aqueous plating solution; whereby a portion of the metal ions of the free metal (i) are reduced to free metal, wherein said reduced free metal displaces surface substrate metal which is oxidized to an ion and complexed with the complexing agent to form a substrate metal ion complex dissolved in the reacted aqueous plating solution at the surface of the substrate metal;   (c) removing the plated substrate metal surface from the the plating solution reservoir; and   (d) removing excess plating solution from the plated substrate metal surface thereby terminating the displacement plating process.   
     
     
       30. The immersion plating process of claim 29 wherein the aqueous plating solution is maintained under an inert atmosphere which is substantially free of oxygen. 
     
     
       31. The immersion plating process of claim 29 wherein the aqueous plating solution contains therein a free metal (iv) which is the free metal of the metal ions (i).